Hi there, I believe i have found the solution - annoyingly I cannot get it to work but thats another matter. I'll share my findings in case it helps someone stumbling across this thread:
"This device is not authorized as a test kindle by the developer of this item. Please contact the developer."
Looks like the error with KUAL was/is caused by "OTA updates" when you connect to wifi. This can be prevented by installing Helper+ through KUAL (without wifi enabled). However, i understand this may not be a permanent fix.
Instead, downgrade your firmware from 4.1.4 back to 4.1.1 as apparently OTA updates arent a problem on this FW... This user had the same experience:
https://www.mobileread.com/forums/sh...d.php?t=352957
I'm calling this problem solved. Hopefully I get around to actually applying this fix and getting it working myself.
Cheers.